1. Field of the Invention
The present invention relates to a process for sealing the rotor of a steam turbine which uses wet geothermal steam under pressure which, in the turbine, passes from a high pressure and temperature at the inlet to a low pressure and temperature at the outlet, passing through intermediate stages of pressure and temperature, the said rotor being provided with a plurality of adjacent labyrinth sealing rings at each side of the turbine and interposed with passages which extend radially of the rotor itself, including at least one step in which a flow of steam is introduced into one of the said radial passages between the sealing rings and made to pass through the labyrinth of at least one of the said sealing rings, being subjected to throttling with a drop in pressure and reduction in temperature, and at least one step in which the steam subjected to throttling is collected through another of the said radial passages.
2. Description of the Prior Art
In steam turbines, according to one prior art technique, sealing of the rotor against the outside of the machine is achieved by feeding high pressure steam to the labyrinths of the sealing rings against the flow of steam tending to escape from the inside of the machine to the outside through these sealing rings.
The steam which is throttled through the labyrinths of the sealing rings is collected and carried to the outside through radial passages or chimneys interposed between the sealing rings.
For this purpose, it is usual to use either live steam or the steam recovered from intermediate stages of the turbine or, in suitable cases, steam lost from high pressure labyrinths and recovered and channeled to labyrinths operating at a lower pressure.
Although the prior art technology is commonly and advantageously applied in the case of turbines using generated steam with accurately controlled purity and quality, in the case of turbines using geothermal steam it involves serious disadvantages owing to dissolved salts contained in the steam which precipitate in the labyrinths of the seals and compromise their effectiveness.
As is known, the flow of steam passing through the labyrinths of the sealing rings is subject to throttling with an associated drop in temperature which, for a given pressure at the outlet of the machine, can make the steam superheated, that is no longer in the presence of its liquid phase.
Though being superheated causes no substantial disadvantages in the case of generated steam, in the case of geothermal steam, rich in dissolved salts, without the liquid phase the salts precipitate and are deposited in the labyrinths of the sealing rings.
In prior art technology, when geothermal steam is used it is necessary to carry out preventive operations to desalinate the steam.
Such desalination, however, has the disadvantage of requiring special apparatus which increases the costs of the plant and causes losses in the maximum pressure of the steam available for introduction into the turbine, when in the case of geothermal steam pressure is originally not that high, generally not exceeding 15 to 20 atmospheres.